1. Field of the Invention
The invention relates to injection of a solution into a hot gas.
The invention is specifically directed to injection of an aqueous solution of urea into an exhaust gas from an engine for example from a diesel engine installed in a vehicle.
2. Description of Related Art
Processes for catalytic reduction of nitrogen oxides in an exhaust gas are known in the art. One is disclosed in DE 4203807, where a urea solution is injected and an evaporator is installed just downstream of the injection. The evaporator is preferably covered by a hydrolysis catalyst to quantitatively decompose the urea and avoid deposit on the downstream reduction catalyst.
The problem of deposition of urea is also dealt with in Japanese patent JP 8206459. Here the deposition of urea is prevented by mixing a urea solution with water upstream of an injection nozzle. Signals from engine control are used to control the mixing ratio, so when a small amount of urea is required during low load of engine, more water is used in order to prevent blocking with urea in the nozzle.
Also JP 2002306929 considers solidification of injected urea into an exhaust gas. The nozzle is housed in a cover, and then downstream of this the hot exhaust gas is introduced, and then a catalyst is needed for decomposition of urea.
Another approach to solving the problem of urea deposit is disclosed in JP 2003278530, where the nozzle is surrounded by a thick wall, and the nozzle tip is surrounded by a cover with a deflecting bottom, after which the urea solution is introduced to the exhaust gas.
To avoid fouling by urea in a selective catalytic reduction unit in exhaust gas in vehicles, U.S. Pat. No. 6,203,770 discloses a process, where the urea is decomposed in a separate chamber before the solution is sprayed into the exhaust gas.
A process for injection of a urea solution into exhaust gas is disclosed in EP 1 052 009. In this process a side stream is conducted through a porous body acting as an evaporator or through a catalytic reactor, where hydrolysis of urea takes place before the side stream is combined with the main stream. This process requires control of the side stream flow and an evaporating and/or hydrolysing device to create a mixture of ammonia and exhaust gas upstream of a selective catalytic reduction of nitrogen oxides.
In WO 2005/103 459 exhaust gas is added a reducing agent by injection into a smooth shell surrounded by spacers pressing against the enlarged shell when hot.
To obtain turbulence in an exhaust gas for improved mixing between an injected reducing agent and the gas, an elaborate device is installed as disclosed in WO 03/036 054.
A common disadvantage of known art is that additional equipment is required in form of evaporator, hydrolysis reactor, water tanks, thick housing, exhaust gas injection device, mixers or separate chamber and some of them even result in risk of cooling down the urea solution, which should have been thermally decomposed. The additional equipment occupies space in an exhaust gas pipe, which is a problem especially in vehicles, where only limited space is available.
The object of the invention is to provide an apparatus for urea solution injection with subsequent evaporation and thermal decomposition of urea without incomplete evaporation or deposition of incomplete decomposed urea and without requirement of additional space in an exhaust gas channel.